See Journeys

Photo by: Flickr/Silke Baron

The awe-inspiring mantis shrimp. Photo by: Flickr/Silke Baron

There is a long-standing puzzle about the wiring of the human eye: why was it wired backwards? The inside-out vertebrate retina has always been presented as an example of inefficient structure locked in by development and evolutionary history. Some recent research has shown that the retina of the eye has been optimized so that the sizes and densities of the glial cells match the colors to which the eye is sensitive. As a result, our color vision during the day is enhanced without having a negative impact on our nighttime vision.

Did you see the dress as white and gold? Or blue and black? How can one dress be perceived in such different and polarized ways? Precisely because people couldn’t agree what color they saw, “the dress” became a polarizing Internet force a few weeks ago. People tended to see the dress either as white-and-gold or blue-and-black, but there was no middle ground and no changing of minds. A short answer as to why people had such divergent opinions about the dress: a matter of color context. (The dress is blue and black.)

This Radiolab episode about colors makes our understanding all the more vivid.

Humans are trichromats—meaning that we have three types of cone cells that are sensitive to the colors blue, green, and red. We also have rods, which help us see in the dark. Animals process light differently—some can detect polarized light (when the wave component of light rotates in a circular motion), some can see ultraviolet light, some have only two kinds of photoreceptors (and are partially colorblind). Birds are tetrachromats (able to see blue, green, red, and ultraviolet) and some birds of prey have sharper vision than humans. Rattlesnakes have low-resolution color vision during the day, but they are also able to sense infrared light. Then there is the mantis shrimp.

Mantis shrimp have the most complex visual system of any animal, with 16 photoreceptors that can detect and see polarized, visible, and UV light. They can perceive depth with one eye and they are able to move each eye independently. They have compound eyes made up of thousands of ommatidia, and in Gonodactylids and Lysiosquillids, six rows of the ommatidia have been modified into something called the mid-band. A group at Washington University in St. Louis is working on a way to detect cancer inspired by the mantis shrimp’s polarized vision and compound eyes.